In the first part of this paper, a comprehensive theoretical study of molecular structure, stability, intramolecular hydrogen bond (IMHB) and [Formula: see text]-electron delocalization ([Formula: see text]-ED) of the enol and thiol tautomers of 3-thioxopropanal (TPA) in the ground state is performed. In this regard, all of the plausible conformations of TPA at M06-2X/6-311[Formula: see text]G(d,p) are optimized and a variety of theoretical levels are employed to identify the global minimum. Our calculations show that E1 is the most stable form that is in contrast to the results of Gonzalez et al. [J Phys Chem 101: 9710, 1997]. In order to elucidate this duality, the IMHB and [Formula: see text]-ED of chelated forms (E1 and T1) have been extensively investigated. So, it is found that both of the IMHB analysis and [Formula: see text]-ED concepts emphasize on the E1, as the global minimum. In the second part of this study, a set of simple electron-withdrawing and electron-donating substituents such as CN, F, Cl, CH3 and NH2 have been considered to evaluate their effects on the IMHB of the first singlet excited state of E1 and T1 at TD-DFT/6–311[Formula: see text]G(d,p) level of theory. According to our analysis, it was found that the IMHB strength of the excited states are much weaker than the ground states. Surprisingly, the IMHB of thiol derivatives is stronger than the enol ones in contrast to the ground state. Furthermore, the substitution effects in the ground and excited states are significantly different. Finally, various linear correlations between the IMHB energies with geometrical, topological and molecular orbital parameters are obtained.
